Use a negative Y value for the gravity Vector3f. pspeed suggests a -20.0f.
There was a forum thread on another solution if that doesn’t work but I cant find it right now.
Search for town.zip, BetterCharacterControl, CharacterControl, gravity, tutorial. I don’t have the time until this afternoon.
Edit: Found the thread but it may not help as there was no follow up by original poster.
They reported different results with different OS so changing Bullet libraries is the only other thing that might help if setting gravity for the physics space itself doesn’t work. Otherwise it may be down to the issue posted by @sgold and still be broken for some it seems.